1. Field of the Invention
The present invention relates to lithium secondary batteries that use a material containing silicon as a negative electrode active material.
2. Description of Related Art
In recent years, lithium secondary batteries using a non-aqueous electrolyte and performing charge-discharge operations by transferring lithium ions between positive and negative electrodes have been utilized as a new type of high power, high energy density secondary battery.
Because of their high energy density, lithium secondary batteries have been widely used as power sources for electronic portable devices related to information technology, such as mobile telephones and notebook computers. It has been expected that, due to further size reduction and advanced functions of these portable devices, load to the lithium secondary batteries used as the device power sources will keep increasing in the future, and demands for higher energy density in the lithium secondary batteries have been very high.
An effective means to achieve higher energy density in a battery is to use a material having a greater energy density as its active material. Recently, it has been proposed and studied to use an alloy material of an element such as Al, Sn, or Si, that intercalates lithium through an alloying reaction with lithium as a negative electrode active material having higher energy density in lithium secondary batteries, in place of graphite, which has been in commercial use.
In the electrode using a material alloyed with lithium as its active material, however, the active material expands and shrinks in volume during the intercalation and deintercalation of lithium, causing the active material to pulverize or peel off from the collector. This leads to the problem of lowering current collection performance in the electrode and degrading a battery's charge-discharge cycle performance.
The present applicant has found that, with a negative electrode using a material containing silicon as its negative electrode active material that is capable of being alloyed with lithium, high current collection performance and good charge-discharge cycle performance are achieved by a negative electrode formed by sintering a mixture layer containing the active material and a binder in a non-oxidizing atmosphere and disposing the mixture layer on a surface of a current collector made of a conductive metal foil having surface irregularities (Japanese Published Unexamined Patent Application No. 2002-260637).
Silicon shows a higher potential during absorbing lithium than negative electrodes with graphite materials and metallic lithium. For this reason, a battery using silicon as its negative electrode active material has a higher positive electrode potential than a battery using metallic lithium or a carbon material as its negative electrode active material, if the same lithium-transition metal composite oxide is used as their positive electrode active material and the batteries are used in the same voltage range. Thus, the reactivity between the positive electrode active material and the non-aqueous electrolyte solution becomes high in the battery using silicon as its negative electrode active material. Since this reaction between the positive electrode active material and the non-aqueous electrolyte solution is a side reaction that does not directly influence the lithium intercalation/deintercalation reaction, the problem of degradation in the charge-discharge cycle performance arises. Moreover, because a higher positive electrode potential during charge causes a greater amount of lithium ions in the positive electrode active material to be deintercalated, the crystal structure of the positive electrode active material becomes unstable and the transition metals in the positive electrode active material tend to dissolve into the electrolyte solution easily. The dissolved metals can deposit on the negative electrode surface during charge and inhibit the lithium intercalation/deintercalation reaction at the negative electrode surface, consequently degrading the battery's charge-discharge cycle performance.